The action of the heart is known to depend on electrical signals within the heart tissue. Occasionally, these electrical signals do not function properly. The maze procedure is a surgical operation for patients with atrial fibrillation that is resistant to medical treatment. In this procedure, incisions are created in the right and left atria to produce an orderly passage of the electrical impulse from the SA node to the atrioventricular node. Blind passageways are also created to suppress reentry cycles. Currently, the lesions may still be created using a traditional cut and sew technique. The scar tissue resulting from the procedure results in a non-conductive lesion.
Ablation of cardiac conduction pathways in the region of tissue where the signals are malfunctioning is now being used to replace the surgical incisions. Ablation is used with other organ tissue, such as the lung, liver, prostate and uterus. Ablation of organic tissue, such as heart, lung or liver tissue, is a technique used in several surgical procedures, for both diagnosis and therapy. In one instance, electrodes at the tips of an electrophysiology ablation device allow the physician to measure electrical signals along the surface of the heart (mapping). In another instance, the physician may also ablate certain tissues using energy (such as radiofrequency energy) conducted to one or more ablation electrodes. Higher levels of energy are used to cut and remove tissue (electrosurgery). Lower levels of energy are used to cause cell damage but leave the structure intact so that electrical pathways are blocked within the tissue.
Sometimes ablation is necessary only at discrete positions along the tissue. This is the case, for example, when ablating accessory pathways, such as in Wolff-Parkinson-White syndrome or AV nodal reentrant tachycardias. At other times, however, ablation is desired along a line, called linear ablation. This is the case for atrial fibrillation, where the aim is to reduce the total mass of contiguous (electrically connected) atrial tissue below a threshold believed to be critical for sustaining multiple reentrant wavelets. Linear lesions are created between electrically non-conductive anatomic landmarks to reduce the contiguous atrial mass.
Linear ablation is currently accomplished in one of several ways. One way is to position the tip portion of the ablation device so that an ablation electrode is located at one end of the target site. This may be done, for example, with an electrode positioned on a “pen-like” device. Then energy is applied to the electrode to ablate the tissue adjacent to the electrode. The tip portion of the electrode is then slid along the tissue to a new position and then the ablation process is repeated. This is sometimes referred to as the “spot burn” technique. This technique is time-consuming (which is not good for the patient) and requires multiple accurate placements of the electrode (which may be difficult for the physician).
Another way of accomplishing linear ablation is to use an ablation device having a series of spaced-apart band or coil electrodes which, after the electrode portion of the ablation device has been properly positioned, are energized simultaneously or one at a time to create the desired lesion. If the electrodes are close enough together the lesions run together sufficiently to create a continuous linear lesion. While this technique eliminates some of the problems associated with the “spot burn” technique, some repositioning of the ablation device may be required to create an adequately long lesion. In addition, it may be difficult to obtain adequate tissue contact pressure for each electrode in a multi-electrode ablation device.
A variety of devices may be used to ablate tissue. Typically, such devices include a conductive tip, which serves as one electrode in an electrical circuit. The electrical circuit is completed via a grounding electrode that may also be on the device or may be coupled to the patient. By controlling the level of energy transmitted to the ablation electrode, the user is able to control the amount of heat generated for the purposes described above. The ablation site may also be irrigated to cool the electrode and create greater lesion depth.
In order to control the level of energy transmitted, the user must monitor the level of energy being transmitted from the electrode. Typical systems for monitoring ablation energy rely on temperature. A thermocouple element is located within the ablation device, generally near the electrode. This temperature-measuring element effectively measures the temperature of the electrode rather than the tissue being ablated. Particularly when the site is being irrigated with a conductive fluid, the temperature of the tissue may differ to some degree from the temperature of the ablation device.
Additionally, water (from within and around the tissue) is present at the ablation site. The heat required to raise the temperature of liquid water by 1° C. is 1.0 kcal/g. However, due to the unique chemical structure of the water molecule, additional heat is required for water to change phase from the liquid to gaseous phase. If the temperature at the ablation site exceeds 100° C., the water will change phase, boil and may result in an audible “steam pop” within the tissue. This pop may damage and even rupture the tissue. Irrigation cooling of the site shifts the location of the “steam pop” even deeper within the tissue, resulting in even greater damage than a superficial pop.
It has been observed that before a “steam pop”, there is a mechanical vibration within the tissue (suspected to be caused by the phase transition of water, which may create microbubbles within the tissue). This vibration transfers to the ablation device. A sensitive enough instrument and a sensitive enough user may perceive this vibration in time to halt ablation, for example, by turning off the energy being delivered to the ablation device. However, due to such reasons as slow human reaction, vibration damping from the device or vibration damping from the tissue, the user is often not able to halt or modify ablation in time to prevent damage.
Thus a means for sensing this vibration in time to halt or modify ablation would be desirable. In addition, a means of automatically halting ablation or modifying the amount of ablation energy being transmitted when this vibration occurs would also be desirable. Moreover, a means of alerting a user to halt or modify ablation would also be desirable.